In general, disc brakes are superior in braking performance and have become very popular. However, a problem exists in that the disc brakes cause vibrations and thus uncomfortable noise at low frequency especially at low speed traveling when braking, sandwiching a brake rotor between brake pads. Such a phenomenon is called brake judder. It has been recently noticed as a new subject in technology requiring analysis and improvement due to a desire to achieve high performance as well as silence in the vehicle.
Although theoretical mechanisms causing brake judder have not been found yet, it is believed that one factor is the accuracy of plane deflection of a pad sliding surface. The accuracy of plane deflection appears as a plane deflection of a side surface of the brake rotor due to accumulation of not only the plane deflection accuracies of the brake rotor itself but the wheel mounting flange on which the brake rotor is mounted, axial deflection of the rolling bearings, accuracy of the inner or outer raceway surfaces and the assembling accuracy of the rolling bearings.
In recent years, measures, for the plane deflection accuracy of the side surface of brake rotor described above, have been taken with satisfying conflicting demands of reduction of size and weight of the wheel bearing apparatus. This improves fuel consumption and increases the rigidity of the wheel bearing apparatus to improve driving stability of the vehicle.
One example of a known wheel bearing apparatus is shown in FIG. 7. This wheel bearing apparatus has an inner member 50, an outer member 60 and double row rolling elements 70, 70. The inner member 50 has a wheel hub 51 and an inner ring 52 press-fit onto a cylindrical portion 53 of the wheel hub 51. One inner raceway surface 51a is formed on the outer circumference of the wheel hub 51. The other inner raceway surface 52a is formed on the outer circumference of the inner ring 52. The wheel hub 51 is integrally formed, on its outer-side end, with a wheel mounting flange 54 to mount a wheel (not shown). Hub bolts 55 are mounted equidistantly on the wheel mounting flange 54.
On the other hand, the outer member 60 is formed, on its outer circumference, with a body mounting flange 61 adapted to be mounted on a body (not shown) of a vehicle. The outer member inner circumference includes double row outer raceway surfaces 60a, 60a. The double row rolling elements (balls) 70, 70 are rollably accommodated between the inner and outer raceway surfaces 51a, 52a and 60a, 60a, via cages 71, 71.
Seals 62, 63 are mounted on both end of the outer member 60 to seal annular spaces between the outer member 60 and the inner member 50. This prevents leakage of lubricating grease confined within the bearing and entry of rain water or dust from the outside into the bearing.
An annular groove 56 is formed on the outer-side surface 54a of the wheel mounting flange 54. Bolt apertures 57 are formed equidistantly along and in the annular groove 56. The hub bolts 55 are secured in the bolt apertures 57 by press-fitting knurled portions 55a of the hub bolts 55 into the bolt apertures 57. A wheel is adapted to be secured on the wheel mounting flange 54, via the brake rotor (not shown), by fastening nuts (not shown) onto the hub bolts 55.
The outer-side surface 54a of the wheel mounting flange 54 is finished processed by grinding after press-fitting of the hub bolts 55. This enables the deformation around the bolt aperture 57 or undulations of the side surface 54a generated by forming the bolt apertures 57 in the annular groove 56 and press-fitting the hub bolts 55 to mainly occur within the annular groove 56 as well as to suppress the influences to the accuracy of plane deflection of the side surface 54a caused by press-fitting of the hub bolts 55 to substantially zero. See, JP2003-154801 A.
However, in the prior art wheel bearing apparatus, when circular apertures (weight reduction apertures) 80 are formed between the hub bolts 55, as shown in FIG. 8(a), muddy water etc. that enters through the weight reduction apertures 80 and often remains in a clearance between the outer-side surface 54a of the wheel mounting flange 54 and a side surface 81a of the brake rotor 81. Thus, rusts is generated on the contact surfaces between the two. As a result, the accuracy of the plane deflection of the side surface 54a of the wheel mounting flange 54 becomes worse. Thus, it is believed that brake judder would be caused as well as the brake rotor 81 strongly fixed to the wheel mounting flange 54, via rust. Thus, disassembly of the brake rotor 81 from the wheel mounting flange 54 would be difficult.
The present disclosure has been achieved with focus on a structure that easily discharges muddy water etc. entering through the weight reduction apertures of the wheel mounting flange while preventing the muddy water etc. from remaining in a clearance space between the wheel mounting flange and the brake rotor. Thus, it is an object of the present disclosure to provide a wheel bearing apparatus that reduces its weight and improves reliability while enhancing the accuracy of the plane deflection of the wheel mounting flange and thus suppressing the generation of brake judder.